K326 / K327 / 2027
Radioactivity overview

Topic 3 of 5

Background radiation and activity

A detector can register radiation even when the particular source being investigated is absent. Separate this background contribution from the source reading.

Radioactive decay is random, so counts fluctuate. A rate means an amount per unit time: equal raw counts collected over different durations need not mean equal rates.

Background is present without the study source

Natural contributions include cosmic radiation, radioactive materials in rocks and soil, radon from the ground, and radionuclides in food and body materials. Artificial sources can also contribute. Background varies with location and conditions.

Removing the study source therefore does not normally make the detector reading zero. Repeated equal-time measurements also need not give exactly the same count.

Convert counts to matching rates

Count rate = number of detected counts / counting timeExamples of units are counts/s and counts/min. State the interval as well as the count.

Five equal one-minute background observations give 18, 21, 19, 22 and 20 counts. Their total is 100 counts in 5 min, so the mean background rate is 100/5 = 20 counts/min.

Compare rates over the correct counting intervals

These are supplied readings. A counter records detected events; its display does not directly give the source's activity. Teal arrows show selected radiation directions, not a count of every emission.

Background only: 100 counts in 5 min

Background only: 100 counts in 5 minThe study source is absent. Dashed teal arrows represent background radiation reaching a detector. A grey signal cable connects the detector to a counter displaying 100 counts, the total from five one-minute background intervals. On a linear zero-to-five-minute scale below, the shaded counting interval lasts five minutes. Dividing 100 counts by five minutes gives an estimated mean background rate of twenty counts per minute. The displayed total is not a rate or an activity in becquerels.BackgroundStudy sourceabsentDetectorCounter total100 countsCounting time / min012345100 / 5 = 20 counts/min

Source plus background: 300 counts in 2 min

Source plus background: 300 counts in 2 minThe study source is present to the left of the same detector. Filled brown dots mark radioactive material; solid teal arrows show selected radiation paths towards the detector. Dashed arrows show that background radiation is still present. The counter displays 300 counts during two minutes. The time bar ends at two on the same zero-to-five-minute scale as the background panel. Dividing 300 by two gives 150 total counts per minute. Subtracting the background rate of twenty gives 130 net source counts per minute. Neither the arrow count nor this uncalibrated detector total measures all nuclear decays.BackgroundStudysourceDetectorCounter total300 countsCounting time / min012345300 / 2 = 150 counts/min

Net source rate = 150 - 20 = 130 counts/min. Subtract the rates, because the two raw totals were collected over different durations.

The same detector is used with the study source absent and present. The intervals are different, so convert each count to a rate before subtracting. A calibrated activity estimate is separate from the detector's raw count display.

Subtract rates, not unmatched counts

300 counts in two minutes with the source present

  1. Total rate: 300/2 = 150 counts/min.
  2. Mean background rate: 100/5 = 20 counts/min.
  3. Estimated source rate: 150 - 20 = 130 counts/min.

Subtracting 100 directly from 300 would compare counts collected over different durations. For this example, take the source activity as approximately constant over the collection intervals.

Optional check A background measurement records 100 counts in 5.0 min. With the source present, the same setup records 300 counts in 2.0 min. What is the estimated net source count rate?
A background measurement records 100 counts in 5.0 min. With the source present, the same setup records 300 counts in 2.0 min. What is the estimated net source count rate?

Activity counts decays, not detector events

The activity A of a radioactive source is the number of nuclear decays per second. Its unit is the becquerel, Bq: 1 Bq means one decay per second.

Here A names a rate. In a nuclide symbol, A instead names nucleon number, which has no unit. Use the quantity and context to distinguish them.

A detector generally registers only a fraction of the source's emissions. Direction, distance, absorbers and detector response matter. A reading of 80 counts/s is therefore not automatically an activity of 80 Bq.

Use a supplied calibration

One recorded source count for every four decays, on average

Suppose a calibration for the long-lived source and unchanged arrangement above gives this response. The net source count rate is 130 counts/min.

  1. Estimate the decay rate: 130 x 4 = 520 decays/min.
  2. Convert to seconds: 520/60 = 8.67 decays/s.
  3. State the activity estimate: about 8.7 Bq.

This result depends on the supplied average calibration. It is not an exact activity obtained merely by changing the label on a counter.

Keep the measurement conditions matched

  • Detector and range: use a suitable detector and counter with a usable rate range and response for the emission. At high rates, some detectors miss events arriving close together.
  • Background: measure it with the study source absent, keeping the detector settings, location and surrounding conditions comparable.
  • Geometry: keep source-detector distance and alignment fixed when comparing a source over time. Changed separation or an added absorber can change counts without any change in source activity.
  • Timing: record the actual collection duration. Longer counting or repeated observations can reduce relative random variation in a steady or background rate.
  • Calibration: the one-in-four factor above applies only to its stated emission, geometry, shielding and settings. A change can invalidate that estimate.

Longer counting is not always better for following a decaying source: a long interval averages over its change. Use intervals short compared with the decay timescale when assigning a rate to a particular time. Repetition cannot correct an inappropriate calibration or changed geometry.